Interferometry is a well known technique for examining a sample using interference between light backscattered from a sample, and light from a reference source. Low-Coherence Interferometry (LCI) and Optical Coherence Tomography (OCT) are examples of methods for examining a sample using backscattered light. LCI is an optical technique that relies on “coherence gating” to provide precise axial positioning of an object in the direction of light propagation. By focusing the light in a sample, a transverse resolution may also be obtained (perpendicular to the optical beam), thus allowing for the collection of information from a finite volume for imaging or optical characterization purposes. OCT is an imaging technique which allows high resolution observation and characterization of tissue microstructure imaging with resolution on the order of micros. This technique measures detailed changes with a few millimeters of a non-transparent tissue structure. One drawback of OCT imaging is the time to acquire a large number of data points necessary to obtain an image over a sufficient area.
Interferometery data is provided by an interferometer which may be configured in any of a number of ways to create various interferometers including, but not limited to, an autocorrelator, a Fizeau interferometer, a Mach Zehnder interferometer, and a Michelson interferometer. FIG. 1 illustrates an exemplary LCI or OCT probe system interferometer that includes a light source 100 which provides light that is propagated through one or more waveguides coupled to a splitter 102 which redirects light from the light source 100 into waveguides that make up at least one reference arm 107 and one or more sample arms 106. In embodiments including more than one sample arm, as shown in FIG. 1, a second splitter 109 or a multiple-output optical switch may be optically coupled to the first splitter 102 to direct light from the light source 100 into a plurality of sample arts 106. In operation, illuminated light from the light source 100 is propagated through the sample arm 106 and the reference arm 107 and is emitted at the distal ends of the sample arm 106 and the reference arm 107. Backscattered light is then collected by the distal ends of the sample arm 106 and the reference arm 107 and propagated back through the sample arm 106 and the reference arm 107 in the opposite direction of the illuminated light. In FIG. 1, light emitted and backscattered light returning to the sample arms 106 are represented by dashed line arrows. Light from the sample arm 106 and the reference arm 107 are then recombined, and a detector 108 is used to convert a light signal from the combined backscattered light into an electronic signal which is passed to a receiver 110 whose output can be obtained and/or measured by a Digital Acquisition Board and a processor 118. In various embodiments, interference between the signal obtained from reference arm and signal from the sample arm may be measured and utilized to characterize morphology, physical nature, composition, and various other properties of a sample in proximity to the distal end of the probe using known methods (See, for example, U.S. patent application Ser. No. 11/039,987, published as 2005/0254060, entitled “Low Coherence Inteferometry for Detecting Plaques” hereby incorporated by reference in its entirety).
Interference may occur between light components having the same polarization. The waveguides 116 of interferometers, as illustrated in FIG. 1, are typically optical fibers which propagate light in two orthogonal polarization states, p-polarization and s-polarization. In many cases, light from the light source 100 injected into optical fibers is polarized to eliminate one of the polarization states. However, as light travels through the optical fiber the polarization state of the light in the optical fiber may change as a result of, for example, birefringence in the optical fiber, fiber handling and/or environmental conditions. Therefore, if light is introduced in the s-polarization state, a certain amount of the light detected may be randomly coupled into the p-polarization state, and if a single detector is used to generate an electronic signal, the amplitude of the signal can vary producing variability in the measurements. In addition, the sample under test itself may be birefringent, causing partial rotation of light propagating through it.
Birefringence caused by passage of light through an optical fiber may be mitigated by creating an interferometer that is polarization insensitive. The interferometer must be initialized or balanced prior to use with a sample that is birefringent. Once the interferometer is balanced, any additional birefringence observed in the data obtained from the backscattered light signal may be attributable to birefringence of the sample. Measured birefringence from the sample may provide significant insight into the characteristics of the sample. For instance, birefringence caused by biological tissues or “tissue birefringence” is a common phenomenon in biological samples examined using LCI or OCT interferometers and mapping these birefringence properties may provide insight into the health of the tissue being examined. Tissue birefringence can be caused by a number of fibrous tissue components, such as, collagen and elastin fibers which arrange themselves in highly anisotropic structures, such as, for example, tendons, ligaments, skin, blood vessels and structures of the eye, brain, and spinal cord and the like. Therefore, without wishing to be bound by theory, difference in the birefringence properties of biological samples can be used to detect disturbances in the regularity of such structures indicative of diseased tissue. For example, birefringence data may be used to determine the depth of a burn on the skin or locate fibrous caps in atherosclerotic vascular tissue.
Embodiments of the invention described herein include receiver architectures designed to eliminate birefringence from an optical signal of an interferometer so that birefringence data from a sample may be obtained.